1. Field of the Invention
The present invention relates to an electrical discharge machine and more particularly to a dielectric fluid filtering apparatus.
2. Description of the Background Art
FIG. 9 is an arrangement diagram of a conventional electrical discharge machine as shown in Japanese Patent Disclosure Publication No. 260934 of 1986, which comprises a machining tank 4 containing a dielectric fluid 3, a workpiece 1 placed on the bottom of the machining tank 4, an electrical discharge machine proper 21 having an electrode 2 servo-fed above the workpiece 1 by a servo head 5, and a dielectric fluid filtering apparatus 30 installed beside the electrical discharge machine 21. A dielectric fluid tank 31 of the dielectric fluid filtering apparatus 30 is sectioned into two machining fluid tanks: a finishing fluid tank 34 and a roughing fluid tank 37. The roughing fluid tank 37 and the machining tank 4 of the electrical discharge machine 21 are connected by a dirty fluid drawing pipe 39 which includes a dirty roughing fluid draining solenoid valve 38, thereby allowing a dirty roughing fluid 40 to be drawn from tank 4, as required. Also, the roughing fluid tank 37 and the machining tank 4 are connected by a clean roughing fluid supplying pipe 43 which includes a clean roughing fluid supplying pump 41, a roughing filter 46 and a clean roughing fluid supplying solenoid valve 42, thereby allowing a clean roughing fluid to be supplied as necessary.
The finishing fluid tank 34 and the machining tank 4 of the electrical discharge machine 21 are connected by a dirty fluid drawing pipe 49 which includes a dirty finishing fluid draining solenoid valve 48, thereby allowing a dirty finishing fluid 50 to be withdrawn as required. Also, the finishing fluid tank 34 and the machining tank 4 are connected by a clean finishing fluid supplying pipe 53 which includes a clean finishing fluid supplying pump 51, a finishing filter 56 and a clean finishing fluid supplying solenoid valve 52, thereby allowing a clean finishing fluid to be supplied as necessary.
In the aforementioned conventional machine, when extremely rough electrical discharge machining is performed on the workpiece 1, wherein, for example, an average machining current of 120A to 500A is applied, the dielectric fluid in the roughing fluid tank 37 is circulated because a large amount of sludge is produced. In this case, the dirty roughing fluid draining solenoid valve 38 and the clean roughing fluid supplying solenoid valve 42 are opened, and the clean roughing fluid supplying pump 41 is operated. The filter 46 is a paper filter that has low filtering accuracy, i.e. it has coarse meshes of approximately 10 to 20 microns. In comparison, when a finishing operation employing a comparatively little current is performed, the roughing fluid tank 37 is not used but the dielectric fluid in the finishing fluid tank 34 is circulated. In this case, the dirty finishing fluid draining solenoid valve 48 and the clean finishing fluid supplying solenoid valve 52 are opened, and the clean finishing fluid supplying pump 51 is operated. The filter 56 is a paper filter that has high filtering accuracy, i.e. it has fine meshes of approximately 3 to 5 microns.
In this conventional electrical discharge machine, the dielectric fluids for roughing and finishing are supplied and filtered separately. However, since the grain diameter of the sludge produced during machining will vary depending on an electrical discharge machining condition, i.e. an average machining current, the conventional system cannot filter the sludge sufficiently. Thus, especially in finishing, some sludge stays in the machining fluid and appears at the machining gap, thereby destabilizing the machining. Also, since the difference between roughing and finishing operations may be difficult to judge, the automatic selection of the proper filtering path may not be possible. Accordingly, often an operator's sense and experience is necessary to decide which type of operation is being conducted and the filtering path of the dielectric fluid normally is switched manually.